1. Field of the Invention
The present invention pertains generally to the use of organic charge transfer materials to produce an optically sensitive medium; and, more particularly, various enhancement mechanisms for providing improved thermal/optical erasure characteristics.
2. Description of the Contemporary and/or Prior Art
With the advent of the information revolution, recent research activities have focused on developing optical storage medium. Currently, optical memory devices rely on photochemical hole burning (PHB) in which a laser pits the material in an effort to store data. An article entitled "Laser Marking of a Thin Organic Film" by J. J. Wrobel et al, Applied Physics Letter 40, (11), June 1, 1982, describes such a technique using a laser beam to burn holes in a thin organic film. Similarly, optical writing on blue, sputtered iridium oxide films is reported by Mabosch et al in Applied Physics Letter 41, July 1, 1982. This technique uses an optical writing mechanism to thermally induce dehydration at temperatures below the melting point of the optical medium. A patent issued to Ovshinsky (U.S. Pat. No. 3,983,542) discloses a material which undergoes a physical change from an amorphous to a crystalline state when exposed to a light beam. An article entitled "Light-induced Phenomena in Dye-polymer Systems" by V. Novotny et al, The Journal of Applied Physics 50 (3), March 1979, describes an optical marking process based on diffusion in a dye-polymer system.
The prior art optical storage systems generally have one overriding disadvantage--prior art optical media is not erasable. As a result, optical storage technology has found little application in computer technology, which requires both read, write and erase functions.
The following U.S. patent and U.S. patent application filed by R. S. Potember, T. O. Poehler and D. O. Cowan, disclose a class of organic charge transfer salts, such as CuTCNQ, which exhibits stable and reproducible switching between an equilibrium, or first state, and a second state, in the presence of an applied electrical field: (1) "Current Controlled Bistable Electrical Organic Thin Film Switching Device (TCNQ)", issued Dec. 1, 1983, U.S. Pat. No. 4,371,883; and, (2) "Method of Fabricating a Current Control Bistable Electrical Organic Thin Film Switching Device (TCNQ)", filed June 7, 1982, Ser. No. 385,523, U.S. Pat. No. 4,507,627. The organic charge transfer salts will undergo a reversible electrochemical topotactic redox reaction in the presence of an applied electric field, thereby switching from a first state to a second state. A detectably different impedance occurs between the equilibrium, or first state, and the switched and second state. In specific, an electrical field is applied across a thin film of CuTCNQ, or an equivalent organic charge transfer salt. When the applied electrical field exceeds a threshold value, the impedance across the thin organic film will drop from a relatively high impedance to a relatively low impedance.
Two papers written by R. S. Potember et al report that when the organic film is electrically switched, the second state has different optical properties from the equilibrium or first state; (1) "The Vibrational and X-ray Photoelectron Spectra of Semiconducting Copper-TCNQ Films" Chemica Scripta, Vol.17, 219-221 (1981); and, (2) "Electrical Switching and Memory Phenomena in Semiconducting Organic Thin Films" American Chemical Society Symposium Series No. 184 (1982). These articles describe infrared spectroscopic means and reference well-known Raman spectroscopic techniques (S. Matsuzaki et al, "Raman Spectra of Conducting TCNQ Salts" Solid State Communications, Vol. 33, pp. 403-405, 1980) for determining if the CuTCNQ film, switched by an AC or DC electric field is in the first or second state. Follow-up work reported by E. I. Kamitsos et al used Raman spectroscopic techniques to verify the electrochemical charge transfer equation described in the above-referenced articles which cause the CuTCNQ salt to switch from the first to second state: "Raman Study of the Mechanism of Electrical Switching in CuTCNQ films" Solid State Communications, Vol. 42, No. 8, pp. 561-565 (1982). The above articles point out that spectroscopic means can be used to discern whether an area of CuTCNQ switched by an applied electrical field is in the first or second state.
Patent application Ser. No. 464,771 filed 2/7/83 by R. S. Potember, T. O. Poehler and R. C. Benson and entitled "Optical Storage and Switching Devices Using Organic Charge Transfer Salts" now U.S. Pat. No. 4,574,366 teaches that in the presence of optical energy the organic charge transfer salt will undergo a redox reaction and switch from a first to a second state; each state having a detectably different optical spectrum. Spectroscopic means can optically determine if a particular spot on the storage medium is in the first (neutral) or second (switched) state. The application further teaches that thermal energy can reverse the electrochemical reaction and return the material to the first (neutral) state. The above application thus teaches an operable optically sensitive medium which can be switched by an optical beam and erased either in sections or on a bit by bit basis.
A paper by R. S. Potember, T. O. Poehler and R. C. Benson entitled "Optical Switching in Semiconductor Organic Thin Films" Applied Physics Letters, Vol. 41 (6), Sept. 15, 1982, and a paper by E. I. Kamitos and W. M. Risen, Jr. entitled "Optically Induced Transformations of Metal TCNQ Materials" Solid State Communications, Vol. 45, No. 2, 1983, discuss the optical switching characteristics of organic charge transfer salts. A U.S. patent application by R. S. Potember and T. O. Poehler entitled "Multistate Optical Switching and Memory Using an Amphoteric Organic Charge Transfer Material" (Ser. No. 603,717, filed 4/25/84) teaches that organic charge transfer materials can be switched into a plurality of states--making multiple bit storage possible at each spot on an optical storage medium.
Although the above references teach the use of organic charge transfer materials in an optically sensitive medium, there has been the need to enhance reproducibility of the erasure process. For an erasable optical storage disc to meet the needs of the current data processing industry, 10.sup.2 -10.sup.6 cycles of bit by bit erasure are desirable.